The invention relates to preparing dual cure laminating adhesives, and laminates including the same.
Flexible packaging materials are widely used in a variety of areas including food packaging. Flexible packaging is often constructed from film laminates in which a layer of laminating adhesive is disposed between a first film (e.g., a polyethylene terephthalate, polyamide or cellophane film) and a second film. Laminates are often used in food packaging because a desired film property cannot be achieved with a single layer or type of film. By combining films having different properties in the form of a laminate, a desired property or combination of properties can be achieved.
Laminates are typically constructed to have a strong cured bond that is resistant to delaminating stresses resulting from differences in modulus of the laminae, as well as to the added stresses imposed in the use of the laminate including exposure to heat, cold and humidity. Substrates used in food laminates often include metallized surfaces and/or have low surface tension, to which it is difficult to form a strong bond
Various adhesives have been developed for use in preparing laminates including solvent based, water based, two component solventless adhesives, and radiation curable adhesives. Organic solvents such as methyl ethyl ketone, ethyl acetate, and alcohols, which are used in some laminating adhesives, tend to be flammable and may pose environmental concerns. Therefore it is desirable to use solventless laminating adhesives. Water-based laminating adhesives, which often include aqueous dispersions of polyurethane and other components, require the expenditure of heat to remove the water.
Because conventional two-component laminating adhesives often require storage for a prolonged period before exhibiting sufficient strength to permit handling and further processing, various efforts have been made to decrease the rate of cure for laminating adhesives. Areas in which recent efforts have been focused include electron beam and ultraviolet (UV) curable adhesive compositions. Although radiation curing can increase the rate of cure, processes that use such techniques do not always produce an adhesive and/or a laminate with suitable properties. Electron beam radiation, for example, can undesirably degrade the heat seal properties of certain films.